1. Field of the Invention
The present invention relates to a printer calibration method and an apparatus therefor, especially for a printer where density or recording history of one color has an influence on density of another color.
2. Background Arts
Generally, color balance and gradation of images printed by a printer vary unexpectedly with many parameters, including conditions of recording paper, environmental conditions, and mechanical conditions of the printer. Therefore, it is necessary to make a calibration of the printer before printing, to achieve a high reproduction.
In a conventional calibration method, a test pattern having an yellow (Y) patch, a magenta (M) patch and a cyan (C) patch, and sometimes a black patch, is printed by the printer to calibrate, and densities of the respective patches are measured by a density measuring device. Then, the printer is calibrated on the basis of the results of measurements so that the respective patches are printed at predetermined set density values. Thereby, the printer should reproduce gray at proper color balance in the entire gradation range.
However, even after the respective color densities are adjusted to the set values, gray is not always reproduced properly. That is, in a printer that prints a full-color image in a color frame sequential fashion, recording conditions of one color can be affected by the previous recording of another color. As a result, even though the same energy is used for recording, density of one color can be different from each other when the color is recorded solely, and when the color is recorded in combination with other colors, for example, for printing a gray image.
Especially, in a thermosensitive color printer that prints an image on a thermosensitive color recording paper by heating the paper directly, coloring characteristic curves of inner thermosensitive coloring layers are affected by the recording on upper thermosensitive coloring layers.
Specifically, electromagnetic rays for fixing the uppermost yellow coloring layer of the thermosensitive color recording material can also fix the next magenta coloring layer to a little degree, and thereby lowers thermal sensitivity of the magenta coloring layer. Since the amount of yellow fixing electromagnetic rays that reach the magenta coloring layer is lessened by yellow pixels previously recorded on the yellow coloring layer, the effect of the yellow fixing rays on the thermal sensitivity of the magenta coloring layer is reduced with an increase in density of previously recorded the yellow pixels.
Consequently, as shown in FIG. 17, even when the same gradation data is used for recording magenta pixels, the magenta coloring layer is colored at higher densities in a gray image portion where the yellow coloring layer is previously colored, as compared to a magenta image portion where the yellow coloring layer is not colored, and thus the largest amount of yellow fixing rays reach the magenta coloring layer.
In addition to the effect of the optical fixing rays, heat energy applied for the thermal recording on the upper coloring layer have an effect on the coloring density of the next color. The heat energy causes a change in surface characteristics of the recording paper, and thus increases heat conductivity of the thermosensitive recording paper. Therefore, the coloring density or the amount of developed pigments of the magenta coloring layer gets higher in the gray image portion as compared to the magenta image portion. The same applies not only to the gray image portion, but also to any colors where pixels are previously recorded on an upper coloring layer.
For these reasons, even through the respective colors are adjusted to the set values with respect to the sole-color patches, consequent gray can not be neutral and have a hue because of the difference in coloring density from the sole-color portion. Therefore, the conventional calibration method is insufficient for those printing systems where density or recording history of one color affects density of another color.
In view of the foregoing, an object of the present invention is to provide an accurate calibration method that is useful for a printer that prints a full-color image on a recording paper in a color frame sequential fashion, and thus for those printing systems where density or recording history of one color affects recording density of another color.
To achieve the above objects, a calibration method of the present invention is comprised of the following steps:
A. printing a pattern predetermined for use in calibration on a recording paper by the printer;
B. measuring color separation densities of the printed pattern;
C. detecting a density difference between the measured density of a first color and a set density of the first color;
D. calculating a first correction amount for correcting recording energy for the first color on the basis of the density difference detected in the step C;
E. estimating densities of second and third colors of the pattern that would be obtained if the pattern is printed after the recording energy for the first color is corrected with the first correction amount;
F. detecting a density difference between the estimated density of the second color and a set density of the second color;
G. calculating a second correction amount for correcting recording energy for the second color on the basis of the density difference detected in the step F;
H. estimating densities of the first and third colors of the pattern that would be obtained if the pattern is printed after the recording energy for the second color is corrected with the second correction amount;
I. detecting a density difference between the last obtained estimated density of the third color and a set density of the third color;
J. calculating a third correction amount for correcting recording energy for the third color on the basis of the density difference detected in the step I;
K. estimating densities of the first and second colors of the pattern that would be obtained if the pattern is printed after the recording energy for the third color is corrected with the third correction amount;
L. detecting a density difference between the last obtained estimated density of the first color and the set density of the first color;
M. checking if the density difference detected in the step L is within a predetermined range;
N. repeating the steps C to M till the density difference detected in step L comes within the predetermined range, while using the last obtained estimated densities of the first and second colors and the set density of the third color in place of the measured densities of the respective colors;
O. memorizing, when the density difference detected in step L comes within the predetermined range, the last obtained recording energy correction amounts for the first to third colors; and
P. correcting the recording energies for the first to third colors with the memorized recording energy correction amounts.
According to the present invention, three color densities of the pattern, preferably a gray pattern, that is printed by the printer are measured, and a recording energy correction amount for one color is determined based on differences between the set densities and the measured densities. Then, density variations of the other two colors that would be resulted from the recording energy correction of the one color are estimated. Thereafter, recording energy correction amounts for these two colors are determined so as to reduce the estimated density variations of these two colors. The same processes are executed repeatedly for each of the three colors so as to obtain recording energy correction amounts for the three colors that minimize the differences between the measured or estimated densities and the set densities of the three colors. In this way, effect of the recording energy correction of one color onto the density variations of the other colors is taken into consideration. Therefore, the color balance and gradation are calibrated accurately to reproduce neutral gray.
Since there is no need for printing and measuring test prints of three colors, the calibration method of the present invention saves the cost and time for calibration.